1. Field of the Invention
This invention relates to an improved system for thermal power generation. More specifically, it relates to that system typically referred to as a reheat cycle, and typically utilized with large nuclear reactors, where the discharge from a first expansion stage is reheated prior to subsequent expansion. This invention places the discharge from the first stage in heat transfer relation with fluid at conditions of a drum component prior to reheating, thereby alleviating the need for complicated flow control apparatus and minimizing thermal cycling of components with changes in load.
2. Description of the Prior Art
A reheat thermal cycle has typically been used in many applications where, due to the type and magnitude of the power source, it is desirable to provide multiple expansion stages. Reheat cycles typically are used in conjunction with superheated fluids and in both once-through and recirculating cycles. A recirculating cycle utilizes recirculation of fluid between an evaporator and a drum. A basic reheat cycle superheats the utilization fluid in a superheater, expands it in a first turbine, reheats in a reheater, sometimes referred to as a resuperheater, and expands it in one or more subsequent turbines. A reheat cycle has been considered the most efficient and advantageous for large nuclear power plant application, particularly so for liquid metal cooled fast breeder reactors. Such reactors generally include a primary circuit circulating a liquid metal coolant such as sodium, an intermediate circuit circulating an intermediate fluid, typically similar to the primary liquid, and a utilization circuit or thermal cycle circulating a vaporizable fluid, such as water. Heat energy is transferred from the primary circuit to the intermediate circuit and then to the utilization circuit, vaporizing and superheating the utilization fluid which drives the series of turbines to produce electric power. The evaporator, drum, and superheater may be separate components or combined units.
Use of a reheat cycle for such application, however, presents concerns as the high heat transfer rate of liquid metals causes severe thermal shock to metal structures, including the superheater and reheater, when system temperature changes occur. These temperature changes occur during plant startup and shutdown, as well as during load changes resulting from increasing or decreasing load demand from the connected electrical system. To mitigate these thermal concerns, several prior art arrangements have been utilized and proposed which, in general, are based upon changing the relative intermediate fluid flow distribution between the superheater and reheater components. These are complicated active control systems which generally include valves and/or pumps operating primarily upon the intermediate fluid circuit, and which must balance flows and sense various parameter changes in all three fluid circuits. The prime control function of such systems is to split and vary the flow of intermediate fluid between the superheater and reheater components. While such control systems reduce the thermal cycling concerns, they do not eliminate it, and further controls such as steam attemporators are typically required. Further, the operational reliability of such control apparatus is a significant concern which can lead to failures and extensive and costly downtime.